1. Field of the Invention
This invention relates to treatment of wastewater produced during food processing operations. It specifically relates to utilizing at least one flow equalization lagoon or basin to perform a controlled hydrolysis stage of biological phosphorus removal, de-nitrification, and partial reduction of 5-day biological oxygen demand (BOD) during flow equalization of 2-to-6-day wastewater inflows to produce substantially uniform 7-day outflows, while maintaining high BOD/TKN and BOD/TP ratios within the influent wastewater of the basin or basins. It further relates to specific recycling steps and a plurality of control methods for maintaining satisfactory anaerobic, anoxic, and aerobic conditions at specific stages of this treatment and at all seasons of the year.
2. Review of the Prior Art
Wastewater treatment, both municipal and industrial, has focussed for years on removal of fats, oil, and grease, suspended solids, and biological oxygen demand (BOD) from wastewater produced by meat processing and other food processing plants, so that lakes and streams into which the treated liquid was discharged would not be de-oxygenated thereby. Then it was realized that the abundance of mineral nutrients from the treated wastewater, combined with sunlight, fostered plant life over animal life, creating eutrophic bodies of water in which the uncontrolled proliferation of various algae and plant species in salt water bays and in fresh water lakes and rivers itself caused de-oxygenation as the dying plants and algae decayed. By E.P.A. estimates, 40 percent of the nation's rivers, 51 percent of the lakes, and 57 percent of the estuaries have been adversely affected by nutrient over-enrichment.
For many years, scientists and engineers attempted to control eutrophication by reducing the amount of ammonia, a poison in itself to aquatic animal life, that was discharged from wastewater treatment plants. Biological aerobic treatment, one of the processes for reducing ammonia, created nitrates and nitrites. Realizing that these are favored plant foods, aerobic treatment of the wastewater was followed by biological anoxic treatment for reducing the nitrates and nitrites to nitrogen gas that harmlessly escaped to the atmosphere.
Next it was realized that denitrification was insufficient because several microbial species can use nitrogen gas from the atmosphere as a source of nitrogen for cellular synthesis. In consequence, phosphorus control was recognized as the key to controlling eutrophication. For years, chemical reactants were added for precipitating phosphorus as sludge. However, this type of phosphorus removal can significantly increase sludge production, typically from 50 percent to 100 percent. In addition to the cost of the chemical reactants, disposal of the additional sludge is expensive.
The poultry industry, for example, has also increased its use of trisodium phosphate (T.S.P.) and other chemical products in order to comply with federal zero-tolerance requirements for eliminating fecal material. The typical poultry plant output of phosphorus is in the range of 15 to 20 mg per liter, but use of T.S.P. in processing operations can increase the phosphorus concentration in the wastewater by 200 percent to 300 percent.
Biological phosphorus removal (BPR) is a simple but effective process that conventionally requires the wastewater to pass through an anaerobic zone and then an aerobic nitrification zone before the mixed liquor is settled in a clarification zone and a portion of the resulting settled sludge is recycled to the aerobic nitrification zone. BPR is improved by inserting an anoxic zone between the anaerobic zone and the aerobic zone so that BPR is combined with biological nitrification and denitrification, as described for municipal wastewater by Dr. Clifford W. Randall, Virginia Polytechnic Institute and State University, in "Theory and Practices of Biological Nutrient Removal."
In the anaerobic zone, which must be adequately mixed, a population of excess phosphorus-storing (polyP) bacteria, such as acinetobacter and other phosphate-accumulating microorganisms that are able to store high amounts of phosphate, up to 10% by weight as polyPhosphate inside the cells, are present. In addition, a suitable substrate, such as soluble carbonaceous Chemical Oxygen Demand (COD) and BOD, in the form of volatile acids, must be present. The 7.3 kcal/mol of energy per mol of adernosine triphosphate (ATP) that is liberated by the hydrolysis of ATP by the polyP bacteria becomes available to the polyP cells, releasing phosphate and forming adenosine diphosphate, ADP. The polyP bacteria use this released energy to polymerize a substrate of organic compounds, such as acetic acid, propionic acid, and other short-chain volatile fatty acids (VFAs), as well as short-chain alcohols, for intracellular storage as polymerized compounds, such as poly-.beta.-hydroxybutyrate (PHB) or poly-.beta.-hydroxyvalerate (PHV).
However, other forms of BOD, such as proteins, cannot be used by the polyP bacteria. Nevertheless, during passage through the basins, a portion of these proteins are gradually broken down by other forms of bacteria into VFAs that gradually become available to the polyP bacteria. This anaerobic breakdown of organic compounds, by enzymes or microorganisms, to simpler products is termed fermentation and occurs naturally in all anaerobic, facultative, and aerobic lagoons. The polyP bacteria actually intercept the breakdown of such organic compounds that would otherwise produce CO.sub.2 and water.
Because the polyP bacteria have no electron acceptors available in the anaerobic zone, they cannot produce new cellular material and multiply in the anaerobic zone, but they can remove certain available organics from solution and sequester them for later utilization in the subsequent aerobic zone where electron acceptors are available. In this aerobic zone, the polyP bacteria have the first opportunity to utilize the BOD so that they have a competitive advantage over the non-polyP bacteria. Thus they can proliferate at a higher rate in the aerobic zone and dominate the activated sludge bacterial population that includes autotrophic nitrifiers using ammonia as their energy source for converting ammonia to nitrite and nitrate.
When the polyP bacteria enter the aerobic zone, they metabolize the stored intracellular compounds for growth and energy. Because excess energy is generated beyond the needs for growth, the polyP bacteria, now much more abundant because of their growth, remove phosphate from solution and store the energy in intracellular phosphate bonds during a "luxury" uptake stage, whereby large quantities of phosphorus are removed from the system in the portion of the sludge that is wasted after clarification. This BPR activity is desirable, but nitrification and de-nitrification are also needed.
The key factor that determines the amount of phosphorus stored in the activated sludge is the amount of readily available organic matter in the anaerobic zone and the absence of electron acceptors such as oxygen and nitrate. The bacteria will preferentially metabolize the organic matter rather than store it if electron acceptors, i.e., dissolved oxygen or oxygen from nitrates and nitrites, are available. Some such electron acceptors are always present in the inflowing wastewater. There must consequently be a large excess of organic matter beyond that needed to deplete the electron acceptors recycled or entrained into the anaerobic zone. In other words, the quantity of stored substrate in the form of organic matter and subsequently the biological removal of phosphorus will be reduced by the quantity of electron acceptors present in the anaerobic zone. U.S. Pat. relating primarily to BPR include U.S. Pat. Nos. 5,288,405; 5,342,522; 5,380,438; 5,480,548; 5,833,856; and 5,853,589.
Biological nitrogen removal (BNR) is another essential wastewater treatment process. It is possible for the BNR process to operate without utilizing an initial anaerobic zone. For example, an initial anoxic zone can be used to cause release of oxygen and depletion of nitrogen as nitrogen gas from broken-down nitrates and nitrites. As a general rule, five parts of BOD per one part of nitrogen (measured as Total Kjeldahl Nitrogen, TKN) must be initially available in order to obtain adequate BNR. This TKN measurement includes NH.sub.3 but does not include nitrates or nitrites.
For example, U.S. Pat. No. 5,611,927 describes a wastewater treatment system that mixes activated sludge with influent wastewater under anoxic conditions in the presence of luxury uptake organisms to cause release of phosphorous compounds into the surrounding wastewater. The wastewater is then aerated in the presence of nitrifying organisms to convert ammonia into nitrate compounds while the luxury uptake organisms take up the phosphorous compounds. The wastewater is next subjected to alternating anoxic and oxic conditions in a plurality of cycles, preferably using two tanks in parallel, to reduce the nitrates to nitrogen gas using denitrifying organisms. The resulting effluent is substantially free of both nitrogen-based and phosphorus-based nutrients.
Although the above-listed U.S. patents also involve BNR activity, U.S. Pat. Nos. 5,401,412 and 5,447,633 are primarily directed to BNR.
Wastewater outflows from a food processing plant may occur during a work week varying from as little as two days, with holidays, to six days. However, equal flows during the entire 7-day week are needed for processing the wastewater. To achieve such equalization, lagoons must be used. They may be anaerobic, facultative, or aerobic, as shown in FIG. 1, wherein Q.sub.5 represents the inflow to a lagoon during a normal work day and Q.sub.7 represents the outflow from the lagoon during any day of the week.
In any case, these lagoons of the prior art significantly reduce BOD of the wastewater while only minimally reducing TKN and TP so that the equalization lagoon effluent has much lower BOD but only slightly reduced nitrogen and phosphorus, nutrients that must not be allowed to enter natural bodies of water. In effect, the microorganisms in the prior art lagoons eat up the BOD during the equalization process but do not eat up the nitrogen and phosphorus. This disproportionate loss of BOD makes it more difficult to remove nitrogen and phosphorus in downstream biological processes. In fact, during prior art processes, BOD must often be added in the form of methanol for denitrification and/or in the form of volatile fatty acids for phosphorus removal.
In industrial poultry processing operations, wastewater pretreatment systems are typically installed and operated upstream of biological final treatment systems. These pretreatment systems include primary and secondary screening to remove meat and feathers, respectively, followed by a chemical treatment and flotation to cause non-soluble solid particles to flocculate together and form a sludge cake containing fat, oil, and grease that floats to the surface and is removed by skimming in a Dissolved Air Flotation (DAF) Cell.
A flow equalization lagoon follows the DAF Cell to provide 24-hour/7-day hydraulic flow equalization and first-stage BOD removal. As shown in FIG. 1, such lagoons typically operate anaerobically but may be operated as facultative lagoons or as aerobic lagoons. They are designed with sufficient capacity to receive raw wastewater inflow generated two-to-six-days/week (allowing for holidays) and to have sufficient equalization volume for wastewater to be discharged or pumped out of the lagoon at a relatively constant rate for 24 hours per day and seven days per week.
These large anaerobic pretreatment lagoons of the prior art are normally unheated and therefore often provide high efficiency BOD pretreatment during the summer months and lower BOD pretreatment efficiency during the winter months. If no DAF Cell is installed upstream of the lagoon, solids and grease accumulation in these lagoons can additionally cause erratic effluent quality and BOD content, especially during the spring and fall seasons when the lagoon temperature is rising and falling, respectively.
Moreover, while prior art anaerobic lagoons remove much BOD, they remove only a small percentage of TKN or ammonia, as indicated in FIG. 2 with typical data, wherein the BOD/TKN ratio is reduced from 12.5 to 2.6 while TKN and TP are reduced only very slightly. Consequently, the BOD/TKN ratio in the lagoon effluent is seasonably variable and is often lower than the desired ratio of at least 5:1 on a weight basis that is necessary for accomplishing downstream denitrification, outside of the lagoon.
In other words, the variable and sometimes excessive BOD removal efficiency, insignificant nitrogen removal efficiency, and low phosphorus removal efficiency that are provided by prior art anaerobic lagoons result in a lagoon effluent having a BOD/TKN ratio (indicating, in effect, carbonaceous oxygen demand/nitrogenous oxygen demand) that is variable, difficult to control, and often too low for high efficiency of total nitrogen removal by nitrification/denitrification in a downstream final treatment system utilizing activated sludge.
These prior art lagoons typically receive wastewater having a nitrogen content, measured as 100 parts of TKN (Total Kjeldahl Nitrogen), that is derived from 20 parts of NH.sub.3 and 80 parts of organic N. After the anaerobic, facultative, and/or aerobic bacteria in these lagoons have been active during flow equalization, the nitrogen content is slightly less than 100 parts, caused by slight nitrogen uptake for biomass growth. BOD is thus significantly reduced, but nitrogen content is substantially unchanged.
As shown in FIG. 1, typical Q.sub.5 (flow per day during a 5-day week) and Q.sub.7 (flow per day during a lagoon-averaged 7-day week) analyses for inflow and outflow streams of prior art anaerobic, facultative, and aerobic lagoons, operated as flow equalization basins, show that BOD is greatly decreased and NH.sub.3 is greatly increased because of deamination (biological activity by various bacterial species) during passage through the lagoon. Deamination is specifically defined as splitting off ammonia from amino acids and proteins by hydrolysis, with formation of the corresponding fatty acids. As illustrated in FIG. 2, such deamination changes most of the BOD to Volatile Fatty Acids (VFAs) and fermentation by other bacteria eventually creates CO.sub.2 and H.sub.2 O and changes a small portion of the TKN to NH.sub.3.
It should be understood that the TKN represents all sources of unoxidized nitrogen, including all proteins, such as blood and muscle particles, as well as NH.sub.3, but does not include nitrates or nitrites. Deamination of these proteins creates most of the NH.sub.3 in all lagoons, as also illustrated in FIG. 2, and continues in the downstream units of both prior art processes and the process of this invention.
Using these typical figures, the BOD/TKN ratio changes from 1500/120 to 300/115, or from 12.5 to 2.6. Because this ratio must be maintained at or above 5.0 on a weight basis in order to have sufficient BOD in subsequent operations for denitrification, nitrification, and luxury uptake of phosphate to take place in adequate quantities, these prior art lagoons generally create highly unsatisfactory downstream conditions during most of the year.
The only U.S. patent that is known to disclose the use of lagoons for biological treatments is No. 4,277,341 of William F. Wise et al. However, the lagoons are described as open bodies of water, not flow equalization systems, and are exemplified by treatment of municipal sewage. They are preferably facultative aerated lagoons, i.e., lagoons in which both aerobic and anaerobic biological treatments take place. Equipped with a plurality of diffused aerators and submerged collectors in the form of horizontally extending tubes having an array of holes on the underside and containing biological reaction media for the growth of nitrifying bacteria, nitrification takes place as the water is pumped through the tubes and out of the lagoon at a controlled velocity gradient until the tubes are periodically backwashed with air. Then any solids collected within the tubes, including a portion of the nitrified film which has formed on the biological reaction media, are discharged and settled as a sludge on the lagoon floor where anaerobic denitrification occurs. No biological phosphorus removal is mentioned.
Nitrogen and phosphorus removal efficiencies at three poultry processing plant wastewater treatment systems were described in a presentation by John H. Reid, P. E., Reid Engineering Company, Inc., for the 1999 Environmental Management Seminar, Atlanta, Ga., Aug. 18-19, 1999. All three of the described facilities had stringent BOD, Total Suspended Solids (TSS), and Ammonia Nitrogen permit limitations. All were operated to achieve both Total Phosphorus (TP) and Total Nitrogen (TN) permit limitations.
Using Cases 1, 2, and 3 to describe these three wastewater treatment facilities, Case 1 was a 1.0 Million Gallon per Day (MGD) design capacity poultry processing plant wastewater treatment system with multi-stage activated sludge reactors providing nitrification and denitrification followed by phosphorous removal by chemical precipitation in the final clarifier with effluent polishing by tertiary filtration. Its permit limits for Ammonia Nitrogen was 3.0 mg/L and for Total Phosphorous was 0.50 mg/L. Although most of the unit arrangements and operations were closely similar to those of the invention, as described hereinafter, phosphorus removal by control of dissolved oxygen and nitrate oxygen in the "7 DAY F.E.B. ANAEROBIC/ANOXIC REACTOR No. 1A" was not utilized. Phosphorus removal was knowingly achieved only by dosage of aluminum sulfate solution into the mixed liquor discharge flow by chemical precipitation in the final clarifier, with augmentation by polymer solution.
Case 2 was a 0.75 MGD design capacity poultry processing plant wastewater system, with multi-stage activated sludge reactors providing nitrification and denitrification, and partial biological phosphorous removal followed by final phosphorous removal by chemical precipitation in the final clarifier. Its permit limits for Ammonia Nitrogen was 3.0 mg/L, for total Nitrogen was 5.0 mg/L, and for total Phosphorous was 0.50 mg/L. Its anaerobic lagoon received no recycles and was operated as a prior art lagoon.
Case 3 was a 0.95 MGD design capacity poultry processing plant wastewater treatment system with multi-stage FEB activated sludge reactors providing denitrification and partial biological phosphorous removal followed by final phosphorous removal by chemical precipitation in the final clarifier. These FEB reactors were started up after June 1999. Its permit limits for Ammonia Nitrogen were 2.7 mg/L and for Total Phosphorous was 2.0 mg/L. Case 3 is the instant invention.
There is consequently a need to utilize flow equalization lagoons for simultaneously performing flow equalizing, the hydrolysis stage of biological phosphorus removal, controlled BOD reduction, and denitrification of nitrates and nitrites from recycled wastewater inflows while avoiding septic conditions and maintaining high BOD/TKN ratios, whereby luxury uptake of phosphate ions, aerobic nitrification of ammonia, and complete aerobic BOD reduction are all that must be done before the mixed liquor is clarified by settling to produce sludge and clarified liquor.
Expressed more simply, the prior art flow equalization lagoon needs to be utilized for efficient biological ATP hydrolysis and nitrogen removal while maintaining a uniform 7-day/24-hour outflow and high BOD/TKN and BOD/TP ratios in the lagoon during all seasons of the year. In view of the numerous chemical and biological reactions, tightly controlled dissolved oxygen contents, selectively directed recycle streams, and vigorous mixing involved with such improvements, "basin", "flow equalization basin (FEB)" or "FEB reactor" are used hereinafter instead of lagoon, and "nitrification reactor" and "anoxic reactor" are used hereinafter for subsequent treatment units, in order to distinguish these units from prior art systems.